1. Field of the Invention
This invention relates to improved low temperature set retarded well cement compositions and methods of cementing subterranean zones in wells using such compositions.
2. Description of the Prior Art
A variety of cementing operations are carried out in the construction and repair of oil, gas and water wells. In such cementing operations, a hydraulic cement is normally mixed with water and other additives to form a pumpable cement composition which is placed into a subterranean zone penetrated by a well bore. After placement in the zone, the cement composition sets into a hard substantially impermeable mass in the zone.
The most common cementing operation performed in the construction of a well is primary cementing whereby a metal pipe string such as casing and/or a liner is placed in the well bore and bonded therein by cement. Primary cementing is accomplished by introducing a cement composition into the annular space between the pipe string and the walls of the well bore, generally by pumping the cement composition downwardly within the pipe string to the bottom thereof and then upwardly into the annulus. Once the cement composition has been placed in the annulus, it is allowed to set into a hard mass therein. The set cement composition bonds the pipe string in the well bore and seals the annulus whereby fluids cannot migrate through the annulus between formations and/or to the surface.
In order to allow a well cement composition time to be mixed on the surface and then pumped into a subterranean zone in a well to be cemented before the cement composition develops gel strength and sets, a set retarding additive is commonly included in the cement composition. Set retarding additives extend the pumping times of cement compositions between mixing and setting whereby they can be placed in desired locations. While the heretofore used set retarding additives successfully extend such pumping times, at low temperatures they often delay the setting of cement compositions for times which are too long, even when only very small quantities of the set retarding additives are utilized. While the cement compositions eventually set, the drilling rig down times while waiting on cement are costly.
Another problem which occurs when a well cement composition used in primary cementing is slow to develop gel strength and to set is the occurrence of pressurized formation fluid flow in the annulus before and after the cement composition sets. Such an occurrence is attributable to the inability of the cement composition to transmit hydrostatic pressure during the transition of the cement composition from a true fluid to a hard, set mass. During the transition phase, initial hydration of the cement composition has begun and the slurry starts to develop static gel strength. While the cement composition has little or no compressive strength, it becomes partially self-supporting which lowers the hydrostatic pressure exerted by the composition on pressurized fluid containing formations penetrated by the well bore. That is, when the cement composition becomes partially self-supporting due to the development of gel strength, any volume reductions in the cement composition result in rapid decreases in the hydrostatic pressure exerted by the composition. Such volume reductions occur as a result of the ongoing hydration reactions and as a result of the loss of part of the fluid phase of the cement composition to permeable subterranean formations. When the pressure exerted by the cement composition falls below the pressure of formation fluids, the formation fluids enter the annulus and flow through the cement composition forming flow passages which remain after the cement composition sets.
While fluid loss reducing additives are commonly included in cement compositions, when the compositions develop gel strength and set over a long period of time after being placed in a location in a well bore, some fluid loss, even though reduced, in combination with the ongoing hydration of the cement compositions can allow pressurized formation fluid migration as described above.
A set retarding additive which has been utilized heretofore and which has rapid gel strength and enhanced compressive strength development is comprised of a copolymer of 2-acrylamido-2-methylpropane sulfonic acid and acrylic acid. The 2-acrylamido-2-methylpropane sulfonic acid is present in the copolymer in an amount in the range of from about 40 to about 60 mole percent with the remainder being acrylic acid. The copolymer set retarding additive is described in U.S. Pat. No. 4,941,536 issued Jul. 17, 1990 to Brothers et al. as well as related U.S. Pat. No. 5,049,288 issued Sep. 17, 1991 to Brothers et al. While the copolymer set retarding additive produces excellent results in cement compositions placed in subterranean zones having temperatures above about 170.degree. F., at temperatures below about 170.degree. F. a cement composition containing the copolymer additive generally does not develop gel strength and compressive strength rapidly enough to prevent excessive down time and/or pressurized formation fluid migration through the cement composition.
Thus, there is a need for improved set retarding cement compositions and methods of using such compositions for cementing subterranean zones in wells at temperatures below about 170.degree. F. whereby after placement, the cement compositions have excellent short-term gel and compressive strength development.